MONITORING OF METHANE CONCENTRATIONS IN A GAS-CHARGED GROUNDWATER AQUIFER USING WATER SAMPLES AND DOWNHOLE TOTAL DISSOLVED GAS PRESSURE (TDGP) SENSORS

Monitoring of groundwater quality in water wells, including methane concentrations, is now legally required in many jurisdictions where oil and gas development activities occur. These data are highly important to ensure that fresh groundwater supplies are not adversely affected by these activities. Because groundwater sampling is time-consuming and labor-intensive, changes in groundwater methane concentrations are typically based on just a few samples. Also, gas sampling is prone to degassing losses, especially in areas with gas-charged groundwater (methane concentrations close to or above 20 mg/L), which can lead to the misinterpretation of any changes. There is a need for more continuous dissolved gas monitoring in methane-rich groundwater, to understand the natural temporal variations in these groundwater systems and to better identify any changes resulting from development. Downhole deployment and continuous data-logging with total dissolved gas pressure (TDGP) sensors appears to be a promising method to aid in this monitoring.

In this study, dissolved gas monitoring was performed for more than two years in three bedrock wells completed into a shale aquifer located above the Utica Shale. Groundwater of this region is typically gas-charged, with methane being the dominant gas. Groundwater samples were collected at surface using a commonly-used semi-closed method every two months, and analyzed for a suite of common gases. TDGP sensors were deployed under hydraulic packers, with data logged at 5 min intervals.

Results of this field study showed that in situ TDGP values were high and quite stable, which fits with the hydrogeological context of the study area, i.e., a low-permeability aquifer with low recharge for which geochemical and microbiological conditions should be close to equilibrium. In contrast, the gas concentrations from groundwater samples were highly variable among sampling events. The percentage of methane among total gas concentrations was, nonetheless, found to be relatively constant. Thus, TDGP sensors represent an interesting tool for long-term dissolved gas monitoring in methane-rich groundwater areas, potentially functioning as a proxy for methane concentrations. These measurements should be complemented with groundwater sampling, among other things for isotopic analyses to help determine its origin.